1. Field of the Invention
The present invention relates to a variable valve gear for an internal combustion engine, capable of changing the lift and the opening and closing timings of an intake valve.
2. Description of the Related Art
Conventionally, a variable valve lift mechanism for changing the lifts of intake and exhaust valves and a variable valve timing mechanism for changing the opening and closing timings (phases) of the valves are known as variable valve gears for internal combustion engines. An increasing number of modern internal combustion engines are equipped with both of these mechanisms in order to further improve the fuel efficiency and output performance.
In one such internal combustion engine provided with both the variable valve lift mechanism and the variable valve timing mechanism, a negative pressure in an intake manifold and a pumping loss are generally reduced or mitigated to improve the fuel efficiency by advancing the closing timing of an intake valve during low-load operation. During high-load operation, on the other hand, the output is enhanced by increasing the lift of the intake valve and hence the intake air amount (Jpn. Pat. Appln. KOKAI Publication No. 2006-97647).
In the technique described in the above patent document, however, the closing timing of the intake valve is delayed as the lift of the valve is increased during the high-load operation. If the closing timing is delayed so that a bottom dead center is approached, the actual compression ratio becomes so high that knocking may be caused. If such knocking occurs, the fuel efficiency is reduced inevitably.